1. Field of the Invention
The present invention relates to high-frequency signal transmission lines and electronic devices, more particularly to a high-frequency signal transmission line preferably for use in high-frequency signal transmission and an electronic device including the same.
2. Description of the Related Art
As an invention relevant to a conventional high-frequency signal transmission line, a high-frequency signal line described in, for example, International Patent Publication WO2012/073591 (see FIG. 12) is known.
The high-frequency signal line described in International Patent Publication WO2012/073591 includes a dielectric element assembly, a signal line, and first and second ground conductors. The dielectric element assembly is formed by laminating a plurality of dielectric sheets. The signal line is provided in the dielectric element assembly. The first and second ground conductors are provided in the dielectric element assembly so as to sandwich the signal line in the direction of lamination. As a result, the signal line and the first and second ground conductors form a stripline structure.
Furthermore, in the high-frequency signal line described in International Patent Publication WO2012/073591, the second ground conductor has a plurality of openings arranged along the signal line. That is, the second ground conductor has a ladder-shaped configuration. Accordingly, little capacitance is created between the signal line and the second ground conductor with the openings, and therefore, it is possible to position the second ground conductor and the signal line close to each other without excessively increasing the capacitance generated between the second ground conductor and the signal line. Thus, the high-frequency signal line described in International Patent Publication WO2012/073591 can be reduced in thickness without causing the characteristic impedance of the signal line to change significantly from a predetermined impedance value.
Incidentally, in the high-frequency signal line described in International Patent Publication WO2012/073591, for the reasons as will be described below, the second ground conductor has bridge portions provided between adjacent openings so as to cross the signal line. The high-frequency signal line is electrically connected at opposite ends to circuit boards by, for example, solder or connectors. Accordingly, the characteristic impedance of the signal line is likely to vary at opposite ends compared to other portions of the signal line. Therefore, high-frequency signal reflection occurs between the opposite ends of the signal line, so that noise with a half wavelength that is equal to the length of the signal line occurs. The frequency of noise with such a long wavelength tends to overlap with the effective bandwidth of an electronic device in which the high-frequency signal line is used.
Therefore, the high-frequency signal line described in International Patent Publication WO2012/073591 has the bridge portions overlapping with the signal line. The characteristic impedance of the signal line is lower in the areas where the signal line overlaps with the bridge portions than in the areas where the signal line does not overlap with the bridge portions. Accordingly, high-frequency signal reflection occurs at the overlapping portions of the signal line and the bridge portions, so that noise with a half wavelength that is equal to the distance between two adjacent bridge portions occurs. The frequency of noise including such a short wavelength is less likely to overlap with the effective frequency of an electronic device in which the high-frequency signal line is used.
However, in the high-frequency signal line described in International Patent Publication WO2012/073591, high-frequency signal reflection occurs at the overlapping portions of the signal line and the bridge portions, resulting in increased insertion loss in the high-frequency signal line.
Therefore, in the high-frequency signal line described in International Patent Publication WO2012/073591, the width of the signal line is narrower in the areas where the signal line overlaps with the bridge portions than in the areas where the signal line does not overlap with the bridge portions. This reduces the capacitance generated between the signal line and the bridge portions, so that the characteristic impedance of the signal line is prevented from becoming excessively low at the overlapping portions of the signal line and the bridge portions. That is, high-frequency signal reflection at the overlapping portions of the signal line and the bridge portions is reduced.
However, in the high-frequency signal line described in International Patent Publication WO2012/073591, since the width of the signal line narrows at the overlapping portions with the bridge portions, the resistance of the signal line becomes high at such overlapping portions. As a result, insertion loss in the high-frequency signal line increases.